1. Field of the Invention
The present invention generally relates to a batch-type etching apparatus for semiconductor wafers and a method for etching semiconductor wafers using the apparatus, and more particularly relates to a semiconductor etching apparatus and method that completely removes contaminating residue found on the surface of a semiconductor wafer after etching.
2. Description of the Related Art
When semiconductor wafers must be processed before a deposition process, wet cleaning, commonly referred to as “RCA cleaning,” has been used for a long time. The process of RCA cleaning involves removal of a silicon spontaneous oxidation film and chemical oxide using diluted hydrofluoric acid (HF) in the final process. Such removal occurs after the semiconductor wafers have been processed for removal of noble metals, hydrocarbons, and other contaminating particles.
In recent years, as devices have become more highly integrated, there have been stronger demands for reducing particle contamination and for decreasing the amount of chemicals used and waste liquid produced. For these reasons, in place of wet etching, vapor etching has been discussed. More particularly, as pretreatment for contact hole filling and gate electrode deposition, a vapor HF etching process, which removes a silicon spontaneous oxidation film, has attracted attention.
In conventional batch-type wet etching devices, because semiconductor wafers are exposed to the atmosphere when transferred into a deposition device after the wet etching process, silicon oxidation films spontaneously re-grow on the semiconductor wafers. The presence of this silicon spontaneous oxidation film on the semiconductor wafer causes increased contact resistance and poor deposition in selective growth process. Additionally, because etching devices partially using vapor HF use etching under atmospheric pressure, such devices cannot be integrated with vacuum load lock type CVD equipment. Thus, etching devices partially using vapor HF also cause silicon oxidation films to spontaneously re-grow on the semiconductor wafers. Although etching devices using plasma have been proposed (Japanese Patent Laid-open No.1987-76632, Japanese Patent Laid-open No.1990-143418), these devices can be integrated only with single-wafer processing type equipment, thereby limiting their use.
Therefore, a batch-type etching device using vapor HF at reduced pressure was proposed. To etch a silicon spontaneous oxidation film at reduced pressure, it is required to cause H2O, CH3OH or CH3COOH gas to adsorb on the surface of a semiconductor wafer to dissociate HF. HF gas dissociates on the semiconductor wafer surface and etches the silicon spontaneous oxidation film by the following reaction:SiO2+4HF→SiF4+2H2OAn etching device using vapor HF at reduced pressure can be easily integrated with CVD equipment possessing a N2 purge box and a vacuum load lock chamber. A problem such as spontaneously re-growing a silicon spontaneous oxidation film does not occur.
There are, however, several problems with such a configuration. In a state in which the air inside a reaction chamber is evacuated after etching is completed, H2O, CH3OH or CH3COOH and reaction by-products remain on the surface of semiconductor wafers. These can be removed to some extent by cyclically filling and purging the reaction chamber with N2 after the etching (a process commonly referred to as “cycle purging”), but they cannot be removed completely. Furthermore, some of these contaminants may be introduced into the CVD equipment, causing further contamination. Additionally, cycle purging takes a long time and therefore lowers productivity.
These residual adsorbates can easily be desorbed and removed by heating. Additionally, an adsorption layer comprising H2O, CH3OH or CH3COOH cannot be formed if the temperature of the semiconductor wafer is high. Consequently, HF cannot dissociate and a silicon spontaneous oxidation film will not be etched. However, heating must be conducted after etching is completed, and such heating takes a significant amount of time because the thermal capacity of the reaction chamber is large. Additionally, after heating it takes a significant amount of time to cool the reaction chamber to room temperature to prepare for etching of the next lot. These prolonged heating and cooling times further lower productivity substantially.